The block diagram in FIG. 1 refers to a “three wire” dimming solution. In the block diagram in FIG. 1, the reference S indicates a light source fed via a driver D connected to three wires, specifically:                a pair of wires 10 that supply power (taking it, for example, from a continuous voltage source), and        a third wire 12 carrying a pulse width modulated (PWM) control signal that commands the dimming function.        
The power supplied via the pair of wires 10 is in fact a continuous power supply and the driver D transfers the power to the source S as a function of the PWM signal on the wire 12, in particular as a function of its duty cycle: the luminosity of the source S is in fact a function of the average intensity of the current flowing through the source S, an intensity that in turn depends on the duty cycle of the control signal.
The block diagram in FIG. 2 refers instead to a system in which the dimming function is realized with a “two wire” system interposing on at least one of the wires of the pair 10 a switch T (for example an electronic switch such as a MOSFET) that is opened and closed using a PWM control signal.
In this case, the power supply of the driver D is no longer continuous but intermittent as schematized in FIG. 3, including two parts indicated respectively with a) and b). The two parts of FIG. 3 are two diagrams that illustrate as a function of a single time scale (x-axis scale, indicated with t), respectively:                the closed, i.e. conductive (“Ton”), or open, i.e. non-conductive (“Toff”), state of the switch T, and        the ideal flow of the supply power to the driver D.        
In the drawing in FIGS. 2 and 3, the dimming function is therefore implemented by controlling, using PWM, the power supply line 10 interrupting in a controlled manner the electrical power to the driver D. By controlling the switching frequency of the switch T such that it is higher than the sensitivity range of the human eye (related to the persistence of the image on the retina), the overall effect achieved is to make the light source S, a function of the average intensity of the current flowing through the source S, dependent on the duty cycle of the PWM signal used to turn the switch T on and off.
Compared to the “three wire” drawing in FIG. 1, the “two wire” drawing in FIG. 2 presents the advantage of doing without one of the wires, which makes the circuit simpler and cheaper. Furthermore, the use of the circuit in FIG. 2 must take into account the presence, at the input of the driver D, of the capacitance C observable as a whole downstream of the switch T, capacitance which may also include at least one capacitor included in the input stage of the driver D.
In operation of the circuit, when the switch T is open, i.e. not conductive, the capacitance C supplies power to the driver D, with the resulting reduction in the voltage present in that capacitance. When the switch T is made conductive again, a voltage step creating an inrush current is applied to the capacitance C. The peak value of this current is nominally limited only by the parasitic resistance of the power supply line including the switch T and the capacitance C and is a function of the width of the aforementioned voltage step, this being the difference between the input voltage from the power source (or the source powering the line 10) and the residual voltage on the capacitance C when the switch T is closed again. This voltage step is therefore a function of the value of the capacitance C and the switching speed (frequency) of the switch T.